FIG. 1 is a circuit diagram showing one example of the conventional charging device for a car. In the figure, the reference numeral 1 designates the armature coil of an alternating current generator mounted on a car and so on (not shown) to be actuated by an internal combustion engine (not shown); 2 designates a field coil provided near the armature coil 1; 3 designates a main rectifying devie for rectification of the armature coil; 4 designates an auxiliary rectifying device; 5 designates a voltage regulating device for controlling voltage between output terminals of the main rectifying device 3 to be a predetermined value by controlling a field current; 6 designates a battery; 7 designates a key-switch connected in a circuit comprising the battery 6, the field coil 2 and the voltage regulating device 5; 8 designates a pilot lamp placed in the vicinity of the key-switch 7 to indicate charging condition of the battery, one terminal of the pilot lamp being connected to the key-switch and the other being connected to the junction between the voltage regulating device 5 and the auxiliary rectifying device 4; and 9 designates an electrical load connected in parallel to the battery 6.
The operation of the conventional device constructed as above-mentioned will be described.
When the engine is in stopped condition, closing of the key-switch 7 flows an initial field current in a circuit of the battery 6.fwdarw.key-switch 7.fwdarw.a pilot lamp 8.fwdarw.field coil 2.fwdarw.voltage regulating device 5.fwdarw.battery 6 with the result that a field ampere-turn is produced in the field coil 2. At the same time, the pilot lamp 8 is lightened to indicate that the battery 6 is not in charging. Under the condition as above-mentioned, when the engine is started by a starting motor, a voltage is induced in the armature coil 1 since the armature coil is excited. When the rotating speed of the engine, that is, the alternating current generator reaches a predetermined value, the field current is supplied through the auxiliary rectifying device 4 and the voltage regulating device 5 whereby the alternating current generator acts as a self-excited generator. Namely, a circuit for field current of the auxiliary rectifying device 4.fwdarw.field coil 2.fwdarw.voltage regulating device 5.fwdarw.ground.fwdarw.main rectifying device 3.fwdarw.auxiliary rectifying device 4 is formed. Further, when the output voltage of the auxiliary rectifying device 4 reaches a predetermined level, the field current is interrupted by the voltage regulating device 5 with the result that the output voltage of the auxiliary rectifying device 4, namely the output voltage of the main rectifying device 3 is kept at a predetermined level. At the same time, the pilot lamp 8 is extinguished to indicate that the battery 6 is in charging.
However, in the conventional device above-mentioned in which a field current is supplied from the output of the generator, particularly of a control system in which a voltage is controlled to become a predetermined level by intermittence of the field current, there appears intermittence of the load in the output of the generator for an amount corresponding to the field current whereby a high surge voltage is produced across output terminals of the rectifier as shown in FIG. 2 when the field current is interrupted, namely the voltage regulating device 5 is actuated. Occurrence of the surge voltage became a principal cause of noise. In FIG. 2, the symbol B designates surge voltage at the time of commutation in the generator.